<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 400px;
            height: 400px;
            background: blue;
            padding-top: 1px;
        }

        .box div {
            width: 100px;
            height: 100px;
            background-color: red;
            /* 理想需求让小div下移 */
            margin-top: 20px;
            /* 出现margin塌陷问题 */
        }
    </style>
</head>

<body>
    <div class="box">
        <div></div>
    </div>
</body>

</html>